Abstract

JSON is a useful data serialization and messaging format.
This specification defines JSON-LD, a JSON-based format to serialize
Linked Data. The syntax is designed to easily integrate into deployed
systems that already use JSON, and provides a smooth upgrade path from
JSON to JSON-LD.
It is primarily intended to be a way to use Linked Data in Web-based
programming environments, to build interoperable Web services, and to
store Linked Data in JSON-based storage engines.

Status of This Document

This section describes the status of this document at the time of its publication. Other
documents may supersede this document. A list of current W3C publications and the latest revision
of this technical report can be found in the W3C technical reports
index at http://www.w3.org/TR/.

This document has been reviewed by W3C Members, by software developers,
and by other W3C groups and interested parties, and is endorsed by the
Director as a W3C Recommendation. It is a stable document and may be
used as reference material or cited from another document. W3C's role in
making the Recommendation is to draw attention to the specification and
to promote its widespread deployment. This enhances the functionality
and interoperability of the Web.

This specification has been developed by the JSON for Linking Data Community Group
before it has been transferred to the RDF Working Group for review,
improvement, and publication along the Recommendation track.
The document contains small editorial changes arising from comments received
during the Proposed Recommendation review; see the
diff-marked version for details.

There are several independent interoperable implementations of this specification. An
implementation report
as of October 2013 is available.

1. Introduction

This section is non-normative.

Linked Data [LINKED-DATA] is a way to create a network of
standards-based machine interpretable data across different documents and
Web sites. It allows an application to start at one piece of Linked Data,
and follow embedded links to other pieces of Linked Data that are hosted on
different sites across the Web.

JSON-LD is a lightweight syntax to serialize Linked Data in
JSON [RFC4627]. Its design allows existing JSON to be interpreted as
Linked Data with minimal changes. JSON-LD is primarily intended to be a
way to use Linked Data in Web-based programming environments, to build
interoperable Web services, and to store Linked Data in JSON-based storage engines. Since
JSON-LD is 100% compatible with JSON, the large number of JSON parsers and libraries
available today can be reused. In addition to all the features JSON provides,
JSON-LD introduces:

and a facility to express one or more directed graphs, such as a social
network, in a single document.

JSON-LD is designed to be usable directly as JSON, with no knowledge of RDF
[RDF11-CONCEPTS]. It is also designed to be usable as RDF, if desired, for
use with other Linked Data technologies like SPARQL. Developers who
require any of the facilities listed above or need to serialize an RDF Graph
or RDF Dataset in a JSON-based syntax will find JSON-LD of interest. People
intending to use JSON-LD with RDF tools will find it can be used as another
RDF syntax, like Turtle [TURTLE]. Complete details of how JSON-LD relates
to RDF are in section 9.Relationship to RDF.

The syntax is designed to not disturb already
deployed systems running on JSON, but provide a smooth upgrade path from
JSON to JSON-LD. Since the shape of such data varies wildly, JSON-LD
features mechanisms to reshape documents into a deterministic structure
which simplifies their processing.

1.1 How to Read this Document

This section is non-normative.

This document is a detailed specification for a serialization of Linked
Data in JSON. The document is primarily intended for the following audiences:

Software developers who want to encode Linked Data in a variety of
programming languages that can use JSON

Software developers who want to convert existing JSON to JSON-LD

Software developers who want to understand the design decisions and
language syntax for JSON-LD

Software developers who want to implement processors and APIs for
JSON-LD

Software developers who want to generate or consume Linked Data,
an RDF graph, or an RDF Dataset in a JSON syntax

A companion document, the JSON-LD Processing Algorithms and API specification
[JSON-LD-API], specifies how to work with JSON-LD at a higher level by
providing a standard library interface for common JSON-LD operations.

To understand the basics in this specification you must first be familiar with
JSON, which is detailed in [RFC4627].

This document almost exclusively uses the term IRI
(Internationalized Resource Indicator)
when discussing hyperlinks. Many Web developers are more familiar with the
URL (Uniform Resource Locator)
terminology. The document also uses, albeit rarely, the URI
(Uniform Resource Indicator)
terminology. While these terms are often used interchangeably among
technical communities, they do have important distinctions from one
another and the specification goes to great lengths to try and use the
proper terminology at all times.

2. Design Goals and Rationale

This section is non-normative.

JSON-LD satisfies the following design goals:

Simplicity

No extra processors or software libraries are necessary to use JSON-LD
in its most basic form. The language provides developers with a very easy
learning curve. Developers only need to know JSON and two
keywords (@context
and @id) to use the basic functionality in JSON-LD.

Compatibility

A JSON-LD document is always a valid JSON document. This ensures that
all of the standard JSON libraries work seamlessly with JSON-LD documents.

Expressiveness

The syntax serializes directed graphs. This ensures that almost
every real world data model can be expressed.

Terseness

The JSON-LD syntax is very terse and human readable, requiring as
little effort as possible from the developer.

Zero Edits, most of the time

JSON-LD ensures a smooth and simple transition from existing
JSON-based systems. In many cases,
zero edits to the JSON document and the addition of one line to the HTTP response
should suffice (see section 6.8 Interpreting JSON as JSON-LD).
This allows organizations that have
already deployed large JSON-based infrastructure to use JSON-LD's features
in a way that is not disruptive to their day-to-day operations and is
transparent to their current customers. However, there are times where
mapping JSON to a graph representation is a complex undertaking.
In these instances, rather than extending JSON-LD to support
esoteric use cases, we chose not to support the use case. While Zero
Edits is a design goal, it is not always possible without adding
great complexity to the language. JSON-LD focuses on simplicity when
possible.

Usable as RDF

JSON-LD is usable by developers as
idiomatic JSON, with no need to understand RDF [RDF11-CONCEPTS].
JSON-LD is also usable as RDF, so people intending to use JSON-LD
with RDF tools will find it can be used like any other RDF syntax.
Complete details of how JSON-LD relates to RDF are in section
9.Relationship to RDF.

3. Terminology

3.1 General Terminology

This document uses the following terms as defined in JSON [RFC4627]. Refer
to the JSON Grammar section in [RFC4627] for formal definitions.

JSON object

An object structure is represented as a pair of curly brackets surrounding
zero or more key-value pairs. A key is a string.
A single colon comes after each key, separating the key from the value.
A single comma separates a value from a following key. In contrast to JSON,
in JSON-LD the keys in an object must be unique.

array

An array structure is represented as square brackets surrounding zero
or more values. Values are separated by commas.
In JSON, an array is an ordered sequence of zero or more values.
While JSON-LD uses the same array representation as JSON,
the collection is unordered by default. While order is
preserved in regular JSON arrays, it is not in regular JSON-LD arrays
unless specifically defined (see section 6.11 Sets and Lists).

string

A string is a sequence of zero or more Unicode characters,
wrapped in double quotes, using backslash escapes (if necessary).

number

A number is similar to that used in most programming languages, except
that the octal and hexadecimal formats are not used and leading zeros
are not allowed.

true and false

Values that are used to express one of two possible boolean states.

null

The null value, which is typically used to clear or forget
data. For example, a key-value pair in the
@context where the value is null explicitly
decouples a term's association with an IRI.
A key-value pair in the body of a JSON-LD document whose
value is null has the same meaning as if the key-value pair
was not defined. If @value, @list, or
@set is set to null in expanded form, then
the entire JSON object is ignored.

3.2 Data Model Overview

This section is non-normative.

Generally speaking, the data model used for JSON-LD is a labeled,
directed graph. The graph contains
nodes, which are connected by
edges. A node is typically data
such as a string, number,
typed values (like dates and times)
or an IRI.
There is also a special class of node called a
blank node, which is typically used to express data that does
not have a global identifier like an IRI.
Blank nodes are identified using a
blank node identifier. This simple data model is incredibly
flexible and powerful, capable of modeling almost any kind of
data. For a deeper explanation of the data model, see
section 7.Data Model.

Developers who are familiar with Linked Data technologies will
recognize the data model as the RDF Data Model. To dive deeper into how
JSON-LD and RDF are related, see
section 9.Relationship to RDF.

3.3 Syntax Tokens and Keywords

JSON-LD specifies a number of syntax tokens and keywords
that are a core part of the language:

@context

Used to define the short-hand names that are used throughout a JSON-LD
document. These short-hand names are called terms and help
developers to express specific identifiers in a compact manner. The
@context keyword is described in detail in
section 5.1 The Context.

4. Conformance

This specification describes the conformance criteria for JSON-LD documents.
This criteria is relevant to authors and authoring tool implementers. As well
as sections marked as non-normative, all authoring guidelines, diagrams, examples,
and notes in this specification are non-normative. Everything else in this
specification is normative.

A JSON-LD document complies with this specification if it follows
the normative statements in appendix 8.JSON-LD Grammar. JSON documents
can be interpreted as JSON-LD by following the normative statements in
section 6.8 Interpreting JSON as JSON-LD. For convenience, normative
statements for documents are often phrased as statements on the properties of the document.

The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT,
RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL in this specification have the
meaning defined in [RFC2119].

5. Basic Concepts

This section is non-normative.

JSON [RFC4627] is a lightweight, language-independent data interchange format.
It is easy to parse and easy to generate. However, it is difficult to integrate JSON
from different sources as the data may contain keys that conflict with other
data sources. Furthermore, JSON has no
built-in support for hyperlinks, which are a fundamental building block on
the Web. Let's start by looking at an example that we will be using for the
rest of this section:

It's obvious to humans that the data is about a person whose
name is "Manu Sporny"
and that the homepage property contains the URL of that person's homepage.
A machine doesn't have such an intuitive understanding and sometimes,
even for humans, it is difficult to resolve ambiguities in such representations. This problem
can be solved by using unambiguous identifiers to denote the different concepts instead of
tokens such as "name", "homepage", etc.

Linked Data, and the Web in general, uses IRIs
(Internationalized Resource Identifiers as described in [RFC3987]) for unambiguous
identification. The idea is to use IRIs
to assign unambiguous identifiers to data that may be of use to other developers.
It is useful for terms,
like name and homepage, to expand to IRIs
so that developers don't accidentally step on each other's terms. Furthermore, developers and
machines are able to use this IRI (by using a web browser, for instance) to go to
the term and get a definition of what the term means. This process is known as IRI
dereferencing.

Leveraging the popular schema.org vocabulary,
the example above could be unambiguously expressed as follows:

In the example above, every property is unambiguously identified by an IRI and all values
representing IRIs are explicitly marked as such by the
@idkeyword. While this is a valid JSON-LD
document that is very specific about its data, the document is also overly verbose and difficult
to work with for human developers. To address this issue, JSON-LD introduces the notion
of a context as described in the next section.

5.1 The Context

This section is non-normative.

When two people communicate with one another, the conversation takes
place in a shared environment, typically called
"the context of the conversation". This shared context allows the
individuals to use shortcut terms, like the first name of a mutual friend,
to communicate more quickly but without losing accuracy. A context in
JSON-LD works in the same way. It allows two applications to use shortcut
terms to communicate with one another more efficiently, but without
losing accuracy.

Simply speaking, a context is used to map terms to
IRIs. Terms are case sensitive
and any valid string that is not a reserved JSON-LD keyword
can be used as a term.

For the sample document in the previous section, a context would
look something like this:

Example 3: Context for the sample document in the previous section

{
"@context":
{
"name": "http://schema.org/name",← This means that 'name' is shorthand for 'http://schema.org/name'
"image": {
"@id": "http://schema.org/image",← This means that 'image' is shorthand for 'http://schema.org/image'
"@type": "@id"← This means that a string value associated with 'image' should be interpreted as an identifier that is an IRI
},
"homepage": {
"@id": "http://schema.org/url",← This means that 'homepage' is shorthand for 'http://schema.org/url'
"@type": "@id"← This means that a string value associated with 'homepage' should be interpreted as an identifier that is an IRI
}
}
}

As the context above shows, the value of a term definition can
either be a simple string, mapping the term to an IRI,
or a JSON object.

Contexts can either be directly embedded
into the document or be referenced. Assuming the context document in the previous
example can be retrieved at http://json-ld.org/contexts/person.jsonld,
it can be referenced by adding a single line and allows a JSON-LD document to
be expressed much more concisely as shown in the example below:

The referenced context not only specifies how the terms map to
IRIs in the Schema.org vocabulary but also
specifies that string values associated with
the homepage and image property
can be interpreted as an IRI ("@type": "@id",
see section 5.2 IRIs for more details). This information allows developers
to re-use each other's data without having to agree to how their data will interoperate
on a site-by-site basis. External JSON-LD context documents may contain extra
information located outside of the @context key, such as
documentation about the terms declared in the
document. Information contained outside of the @context value
is ignored when the document is used as an external JSON-LD context document.

In JSON-LD documents,
contexts may also be specified inline.
This has the advantage that documents can be processed even in the
absence of a connection to the Web. Ultimately, this is a modeling decision
and different use cases may require different handling.

This section only covers the most basic features of the JSON-LD
Context. More advanced features related to the JSON-LD Context are covered
in section 6.Advanced Concepts.

5.2 IRIs

This section is non-normative.

IRIs (Internationalized Resource Identifiers
[RFC3987]) are fundamental to Linked Data as that is how most
nodes and properties
are identified. In JSON-LD, IRIs may be represented as an
absolute IRI or a relative IRI. An
absolute IRI is defined in [RFC3987] as containing a
scheme along with path and optional query and
fragment segments. A relative IRI is an IRI
that is relative to some other absolute IRI.
In JSON-LD all relative IRIs are resolved
relative to the base IRI.

A string is interpreted as an IRI when it is the
value of an @id member:

Example 6: Values of @id are interpreted as IRI

{
...
"homepage": { "@id": "http://example.com/" }
...
}

Values that are interpreted as IRIs, can also be
expressed as relative IRIs. For example,
assuming that the following document is located at
http://example.com/about/, the relative IRI../ would expand to http://example.com/ (for more
information on where relative IRIs can be
used, please refer to section 8.JSON-LD Grammar).

In the example above, since the value http://manu.sporny.org/
is expressed as a JSON string, the type coercion
rules will transform the value into an IRI when processing the data.
See section 6.5 Type Coercion for more
details about this feature.

In summary, IRIs can be expressed in a variety of
different ways in JSON-LD:

An IRI is generated for the string value specified using
@id or @type.

An IRI is generated for the string value of any key for which there
are coercion rules that contain an @type key that is
set to a value of @id or @vocab.

This section only covers the most basic features associated with IRIs
in JSON-LD. More advanced features related to IRIs are covered in
section 6.Advanced Concepts.

5.3 Node Identifiers

This section is non-normative.

To be able to externally reference nodes
in a graph, it is important that
nodes have an identifier. IRIs
are a fundamental concept of Linked Data, for
nodes to be truly linked, dereferencing the
identifier should result in a representation of that node.
This may allow an application to retrieve further information about a
node.

This document uses an empty @id, which resolves to the document base.
However, if the document is moved to a different location, the IRI would change.
To prevent this without having to use an absolute IRI, a context
may define an @base mapping, to overwrite the base IRI for the document.

Please note that the @base will be ignored if used in
external contexts.

6.2 Default Vocabulary

This section is non-normative.

At times, all properties and types may come from the same vocabulary. JSON-LD's
@vocab keyword allows an author to set a common prefix to be used
for all properties and types that do not match a term and are neither
a compact IRI nor an absolute IRI (i.e., they do
not contain a colon).

If @vocab is used but certain keys in an
object should not be expanded using
the vocabulary IRI, a term can be explicitly set
to null in the context. For instance, in the
example below the databaseId member would not expand to an
IRI.

6.3 Compact IRIs

This section is non-normative.

A compact IRI is a way of expressing an IRI
using a prefix and suffix separated by a colon (:).
The prefix is a term taken from the
active context and is a short string identifying a
particular IRI in a JSON-LD document. For example, the
prefix foaf may be used as a short hand for the
Friend-of-a-Friend vocabulary, which is identified using the IRIhttp://xmlns.com/foaf/0.1/. A developer may append
any of the FOAF vocabulary terms to the end of the prefix to specify a short-hand
version of the absolute IRI for the vocabulary term. For example,
foaf:name would be expanded to the IRIhttp://xmlns.com/foaf/0.1/name.

In the example above, foaf:name expands to the IRIhttp://xmlns.com/foaf/0.1/name and foaf:Person expands
to http://xmlns.com/foaf/0.1/Person.

Prefixes are expanded when the form of the value
is a compact IRI represented as a prefix:suffix
combination, the prefix matches a term defined within the
active context, and the suffix does not begin with two
slashes (//). The compact IRI is expanded by
concatenating the IRI mapped to the prefix to the (possibly empty)
suffix. If the prefix is not defined in the active context,
or the suffix begins with two slashes (such as in http://example.com),
the value is interpreted as absolute IRI instead. If the prefix is an
underscore (_), the value is interpreted as blank node identifier
instead.

It's also possible to use compact IRIs within the context as shown in the
following example:

Both examples above would generate the value
2010-05-29T14:17:39+02:00 with the type
http://www.w3.org/2001/XMLSchema#dateTime. Note that it is
also possible to use a term or a compact IRI to
express the value of a type.

A node type specifies the type of thing
that is being described, like a person, place, event, or web page. A
value type specifies the data type of a particular value, such
as an integer, a floating point number, or a date.

The first use of @type associates a node type
(http://schema.org/BlogPosting) with the node,
which is expressed using the @idkeyword.
The second use of @type associates a value type
(http://www.w3.org/2001/XMLSchema#dateTime) with the
value expressed using the @valuekeyword. As a
general rule, when @value and @type are used in
the same JSON object, the @typekeyword is expressing a value type.
Otherwise, the @typekeyword is expressing a
node type. The example above expresses the following data:

Subject

Property

Value

Value Type

http://example.org/posts#TripToWestVirginia

http://www.w3.org/1999/02/22-rdf-syntax-ns#type

http://schema.org/BlogPosting

-

http://example.org/posts#TripToWestVirginia

http://purl.org/dc/terms/modified

2010-05-29T14:17:39+02:00

http://www.w3.org/2001/XMLSchema#dateTime

6.5 Type Coercion

This section is non-normative.

JSON-LD supports the coercion of values to particular data types.
Type coercion allows someone deploying JSON-LD to coerce the incoming or
outgoing values to the proper data type based on a mapping of data type IRIs to
terms. Using type coercion, value representation is preserved without requiring
the data type to be specified with each piece of data.

Type coercion is specified within an expanded term definition
using the @type key. The value of this key expands to an IRI.
Alternatively, the keywords@id or @vocab may be used
as value to indicate that within the body of a JSON-LD document, a string value of a
term coerced to @id or @vocab is to be interpreted as an
IRI. The difference between @id and @vocab is how values are expanded
to absolute IRIs. @vocab first tries to expand the value
by interpreting it as term. If no matching term is found in the
active context, it tries to expand it as compact IRI or absolute IRI
if there's a colon in the value; otherwise, it will expand the value using the
active context's vocabulary mapping, if present, or by interpreting it
as relative IRI. Values coerced to @id in contrast are expanded as
compact IRI or absolute IRI if a colon is present; otherwise, they are interpreted
as relative IRI.

Terms or compact IRIs used as the value of a
@type key may be defined within the same context. This means that one may specify a
term like xsd and then use xsd:integer within the same
context definition.

The example below demonstrates how a JSON-LD author can coerce values to
typed values and IRIs.

In this case the @id definition in the term definition is optional.
If it does exist, the compact IRI or IRI representing
the term will always be expanded to IRI defined by the @id
key—regardless of whether a prefix is defined or not.

Type coercion is always performed using the unexpanded value of the key. In the
example above, that means that type coercion is done looking for foaf:age
in the active context and not for the corresponding, expanded
IRIhttp://xmlns.com/foaf/0.1/age.

Note

Keys in the context are treated as terms for the purpose of
expansion and value coercion. At times, this may result in multiple representations for the same expanded IRI.
For example, one could specify that dog and cat both expanded to http://example.com/vocab#animal.
Doing this could be useful for establishing different type coercion or language specification rules. It also allows a compact IRI (or even an
absolute IRI) to be defined as something else entirely. For example, one could specify that
the termhttp://example.org/zoo should expand to
http://example.org/river, but this usage is discouraged because it would lead to a
great deal of confusion among developers attempting to understand the JSON-LD document.

6.6 Embedding

This section is non-normative.

Embedding is a JSON-LD feature that allows an author to
use node objects as
property values. This is a commonly used mechanism for
creating a parent-child relationship between two nodes.

The example shows two nodes related by a property from the first node:

Example 26: Embedding a node object as property value of another node object

A node object, like the one used above, may be used in
any value position in the body of a JSON-LD document.

6.7 Advanced Context Usage

This section is non-normative.

Section 5.1The Context introduced the basics of what makes
JSON-LD work. This section expands on the basic principles of the
context and demonstrates how more advanced use cases can
be achieved using JSON-LD.

In general, contexts may be used at any time a
JSON object is defined. The only time that one cannot
express a context is inside a context definition itself. For example, a
JSON-LD document may use more than one context at different
points in a document:

In the example above, the nameterm is overridden
in the more deeply nested details structure. Note that this is
rarely a good authoring practice and is typically used when working with
legacy applications that depend on a specific structure of the
JSON object. If a term is redefined within a
context, all previous rules associated with the previous definition are
removed. If a term is redefined to null,
the term is effectively removed from the list of
terms defined in the active context.

Multiple contexts may be combined using an array, which is processed
in order. The set of contexts defined within a specific JSON object are
referred to as local contexts. The
active context refers to the accumulation of
local contexts that are in scope at a
specific point within the document. Setting a local context
to null effectively resets the active context
to an empty context. The following example specifies an external context
and then layers an embedded context on top of the external context:

When possible, the context definition should be put
at the top of a JSON-LD document. This makes the document easier to read and
might make streaming parsers more efficient. Documents that do not have the
context at the top are still conformant JSON-LD.

Note

To avoid forward-compatibility issues, terms
starting with an @ character are to be avoided as they
might be used as keywords in future versions
of JSON-LD. Terms starting with an @ character that are not
JSON-LD 1.0 keywords are treated as any other term, i.e.,
they are ignored unless mapped to an IRI. Furthermore, the use of
empty terms ("") is not allowed as
not all programming languages are able to handle empty JSON keys.

6.8 Interpreting JSON as JSON-LD

Ordinary JSON documents can be interpreted as JSON-LD by referencing a JSON-LD
context document in an HTTP Link Header. Doing so allows JSON to
be unambiguously machine-readable without requiring developers to drastically
change their documents and provides an upgrade path for existing infrastructure
without breaking existing clients that rely on the application/json
media type or a media type with a +json suffix as defined in
[RFC6839].

In order to use an external context with an ordinary JSON document, an author
MUST specify an IRI to a valid JSON-LD document in
an HTTP Link Header [RFC5988] using the http://www.w3.org/ns/json-ld#context
link relation. The referenced document MUST have a top-level JSON object.
The @context subtree within that object is added to the top-level
JSON object of the referencing document. If an array
is at the top-level of the referencing document and its items are
JSON objects, the @context
subtree is added to all array items. All extra information located outside
of the @context subtree in the referenced document MUST be
discarded. Effectively this means that the active context is
initialized with the referenced external context. A response MUST NOT
contain more than one HTTP Link Header [RFC5988] using the
http://www.w3.org/ns/json-ld#context link relation.

The following example demonstrates the use of an external context with an
ordinary JSON document:

Please note that JSON-LD documents
served with the application/ld+json
media type MUST have all context information, including references to external
contexts, within the body of the document. Contexts linked via a
http://www.w3.org/ns/json-ld#context HTTP Link Header MUST be
ignored for such documents.

6.9 String Internationalization

This section is non-normative.

At times, it is important to annotate a string
with its language. In JSON-LD this is possible in a variety of ways.
First, it is possible to define a default language for a JSON-LD document
by setting the @language key in the context:

The example above would associate the ja language
code with the two strings花澄 and 科学者.
Languages codes are defined in [BCP47]. The default language applies to all
string values that are not type coerced.

To clear the default language for a subtree, @language can
be set to null in a local context as follows:

The example above would associate 忍者 with the specified default
language code ja, Ninja with the language code
en, and Nindža with the language code cs.
The value of name, Yagyū Muneyoshi wouldn't be
associated with any language code since @language was reset to
null in the expanded term definition.

Just as in the example above, systems often need to express the value of a
property in multiple languages. Typically, such systems also try to ensure that
developers have a programmatically easy way to navigate the data structures for
the language-specific data. In this case, language maps
may be utilized.

The example above expresses exactly the same information as the previous
example but consolidates all values in a single property. To access the
value in a specific language in a programming language supporting dot-notation
accessors for object properties, a developer may use the
property.language pattern. For example, to access the occupation
in English, a developer would use the following code snippet:
obj.occupation.en.

Third, it is possible to override the default language by using a
value object:

6.10 IRI Expansion within a Context

This section is non-normative.

In general, normal IRI expansion rules apply
anywhere an IRI is expected (see section 5.2 IRIs). Within
a context definition, this can mean that terms defined
within the context may also be used within that context as long as
there are no circular dependencies. For example, it is common to use
the xsd namespace when defining typed values:

In this example, the compact IRI form is used in two different
ways.
In the first approach, foaf:age declares both the
IRI for the term (using short-form) as well as the
@type associated with the term. In the second
approach, only the @type associated with the term is
specified. The full IRI for
foaf:homepage is determined by looking up the foafprefix in the
context.

In order for the absolute IRI to match above, the absolute IRI
needs to be used in the JSON-LD document. Also note that foaf:homepage
will not use the { "@type": "@id" } declaration because
foaf:homepage is not the same as http://xmlns.com/foaf/0.1/homepage.
That is, terms are looked up in a context using
direct string comparison before the prefix lookup mechanism is applied.

Note

While it is possible to define a compact IRI, or
an absolute IRI to expand to some other unrelated IRI
(for example, foaf:name expanding to
http://example.org/unrelated#species), such usage is strongly
discouraged.

The only exception for using terms in the context is that
circular definitions are not allowed. That is,
a definition of term1 cannot depend on the
definition of term2 if term2 also depends on
term1. For example, the following context definition
is illegal:

Example 41: Illegal circular definition of terms within a context

{
"@context":
{
"term1": "term2:foo",
"term2": "term1:bar"
},
...
}

6.11 Sets and Lists

This section is non-normative.

A JSON-LD author can express multiple values in a compact way by using
arrays. Since graphs do not describe ordering for links
between nodes, arrays in JSON-LD do not provide an ordering of the
contained elements by default. This is exactly the opposite from regular JSON
arrays, which are ordered by default. For example, consider the following
simple document:

This describes the use of this array as being ordered,
and order is maintained when processing a document. If every use of a given multi-valued
property is a list, this may be abbreviated by setting @container
to @list in the context:

List of lists in the form of list objects
are not allowed in this version of JSON-LD. This decision was made due to the
extreme amount of added complexity when processing lists of lists.

While @list is used to describe ordered lists,
the @set keyword is used to describe unordered sets.
The use of @set in the body of a JSON-LD document
is optimized away when processing the document, as it is just syntactic
sugar. However, @set is helpful when used within the context
of a document.
Values of terms associated with an @set or @list container
are always represented in the form of an array,
even if there is just a single value that would otherwise be optimized to
a non-array form in compact form (see
section 6.18 Compacted Document Form). This makes post-processing of
JSON-LD documents easier as the data is always in array form, even if the
array only contains a single value.

6.12 Reverse Properties

This section is non-normative.

JSON-LD serializes directed graphs. That means that
every property points from a node to another node
or value. However, in some cases, it is desirable
to serialize in the reverse direction. Consider for example the case where a person
and its children should be described in a document. If the used vocabulary does not
provide a childrenproperty but just a parentproperty, every node representing a child would have to
be expressed with a property pointing to the parent as in the following
example.

6.13 Named Graphs

This section is non-normative.

At times, it is necessary to make statements about a graph
itself, rather than just a single node. This can be done by
grouping a set of nodes using the @graphkeyword. A developer may also name data expressed using the
@graphkeyword by pairing it with an
@idkeyword as shown in the following example:

The example above expresses a named graph that is identified
by the IRIhttp://example.org/graphs/73. That
graph is composed of the statements about Manu and Gregg. Metadata about
the graph itself is expressed via the generatedAt property,
which specifies when the graph was generated. An alternative view of the
information above is represented in table form below:

Graph

Subject

Property

Value

Value Type

http://example.org/graphs/73

http://www.w3.org/ns/prov#generatedAtTime

2012-04-09

http://www.w3.org/2001/XMLSchema#date

http://example.org/graphs/73

http://manu.sporny.org/about#manu

http://www.w3.org/2001/XMLSchema#type

http://xmlns.com/foaf/0.1/Person

http://example.org/graphs/73

http://manu.sporny.org/about#manu

http://xmlns.com/foaf/0.1/name

Manu Sporny

http://example.org/graphs/73

http://manu.sporny.org/about#manu

http://xmlns.com/foaf/0.1/knows

http://greggkellogg.net/foaf#me

http://example.org/graphs/73

http://greggkellogg.net/foaf#me

http://www.w3.org/2001/XMLSchema#type

http://xmlns.com/foaf/0.1/Person

http://example.org/graphs/73

http://greggkellogg.net/foaf#me

http://xmlns.com/foaf/0.1/name

Gregg Kellogg

http://example.org/graphs/73

http://greggkellogg.net/foaf#me

http://xmlns.com/foaf/0.1/knows

http://manu.sporny.org/about#manu

When a JSON-LD document's top-level structure is an
object that contains no other
properties than @graph and
optionally @context (properties that are not mapped to an
IRI or a keyword are ignored),
@graph is considered to express the otherwise implicit
default graph. This mechanism can be useful when a number
of nodes exist at the document's top level that
share the same context, which is, e.g., the case when a
document is flattened. The
@graph keyword collects such nodes in an array
and allows the use of a shared context.

6.14 Identifying Blank Nodes

This section is non-normative.

At times, it becomes necessary to be able to express information without
being able to uniquely identify the node with an IRI.
This type of node is called a blank node. JSON-LD does not require
all nodes to be identified using @id. However, some graph topologies
may require identifiers to be serializable. Graphs containing loops, e.g., cannot
be serialized using embedding alone, @id must be used to connect the nodes.
In these situations, one can use blank node identifiers,
which look like IRIs using an underscore (_)
as scheme. This allows one to reference the node locally within the document, but
makes it impossible to reference the node from an external document. The
blank node identifier is scoped to the document in which it is used.

The example above contains information about two secret agents that cannot be identified
with an IRI. While expressing that agent 1 knows agent 2
is possible without using blank node identifiers,
it is necessary to assign agent 1 an identifier so that it can be referenced
from agent 2.

It is worth nothing that blank node identifiers may be relabeled during processing.
If a developer finds that they refer to the blank node more than once,
they should consider naming the node using a dereferenceable IRI so that
it can also be referenced from other documents.

6.15 Aliasing Keywords

This section is non-normative.

Each of the JSON-LD keywords,
except for @context, may be aliased to application-specific
keywords. This feature allows legacy JSON content to be utilized
by JSON-LD by re-using JSON keys that already exist in legacy documents.
This feature also allows developers to design domain-specific implementations
using only the JSON-LD context.

In the example above, the @id and @typekeywords have been given the aliases
url and a, respectively.

Since keywords cannot be redefined, they can also not be aliased to
other keywords.

6.16 Data Indexing

This section is non-normative.

Databases are typically used to make access to
data more efficient. Developers often extend this sort of functionality into
their application data to deliver similar performance gains. Often this
data does not have any meaning from a Linked Data standpoint, but is
still useful for an application.

JSON-LD introduces the notion of index maps
that can be used to structure data into a form that is
more efficient to access. The data indexing feature allows an author to
structure data using a simple key-value map where the keys do not map
to IRIs. This enables direct access to data
instead of having to scan an array in search of a specific item.
In JSON-LD such data can be specified by associating the
@indexkeyword with a
@container declaration in the context:

In the example above, the postterm has
been marked as an index map. The en and
de keys will be ignored semantically, but preserved
syntactically, by the JSON-LD Processor. This allows a developer to
access the German version of the post using the
following code snippet: obj.post.de.

6.17 Expanded Document Form

This section is non-normative.

The JSON-LD Processing Algorithms and API specification [JSON-LD-API]
defines a method for expanding a JSON-LD document.
Expansion is the process of taking a JSON-LD document and applying a
@context such that all IRIs, types, and values
are expanded so that the @context is no longer necessary.

JSON-LD's media type defines a
profile parameter which can be used to signal or request
expanded document form. The profile URI identifying expanded document
form is http://www.w3.org/ns/json-ld#expanded.

6.18 Compacted Document Form

This section is non-normative.

The JSON-LD Processing Algorithms and API specification [JSON-LD-API] defines
a method for compacting a JSON-LD document. Compaction is the process
of applying a developer-supplied context to shorten IRIs
to terms or compact IRIs
and JSON-LD values expressed in expanded form to simple values such as
strings or numbers.
Often this makes it simpler to work with document as the data is expressed in
application-specific terms. Compacted documents are also typically easier to read
for humans.

JSON-LD's media type defines a
profile parameter which can be used to signal or request
compacted document form. The profile URI identifying compacted document
form is http://www.w3.org/ns/json-ld#compacted.

6.19 Flattened Document Form

This section is non-normative.

The JSON-LD Processing Algorithms and API specification [JSON-LD-API] defines
a method for flattening a JSON-LD document. Flattening collects all
properties of a node in a single JSON object and labels
all blank nodes with
blank node identifiers.
This ensures a shape of the data and consequently may drastically simplify the code
required to process JSON-LD in certain applications.

6.20 Embedding JSON-LD in HTML Documents

This section is non-normative.

HTML script tags can be used to embed blocks of data in documents.
This way, JSON-LD content can be easily embedded in HTML by placing
it in a script element with the type attribute set to
application/ld+json.

7. Data Model

JSON-LD is a serialization format for Linked Data based on JSON.
It is therefore important to distinguish between the syntax, which is
defined by JSON in [RFC4627], and the data model which is
an extension of the RDF data model [RDF11-CONCEPTS]. The precise
details of how JSON-LD relates to the RDF data model are given in
section 9. Relationship to RDF.

To ease understanding for developers unfamiliar with the RDF model, the
following summary is provided:

A graph
is a labeled directed graph, i.e., a set of nodes
connected by edges.

Every edge has a direction associated with it and is labeled with
an IRI or a blank node identifier. Within the JSON-LD syntax
these edge labels are called
properties.
Whenever practical, an edgeSHOULD be labeled with an IRI.

A graphMUST NOT contain unconnected nodes,
i.e., nodes which are not connected by an edge to any other node.

An IRI
(Internationalized Resource Identifier) is a string that conforms to the syntax
defined in [RFC3987]. IRIs used within a
graphSHOULD return a Linked Data document describing
the resource denoted by that IRI when being dereferenced.

JSON-LD allows keywords to be aliased
(see section 6.15 Aliasing Keywords for details). Whenever a keyword is
discussed in this grammar, the statements also apply to an alias for
that keyword. For example, if the active context
defines the termid as an alias for @id,
that alias may be legitimately used as a substitution for @id.
Note that keyword aliases are not expanded during context
processing.

8.1 Terms

To avoid forward-compatibility issues, a termSHOULD NOT start
with an @ character as future versions of JSON-LD may introduce
additional keywords. Furthermore, the term MUST NOT
be an empty string ("") as not all programming languages
are able to handle empty JSON keys.

8.3 Value Objects

A value objectMUST be a JSON object containing the
@value key. It MAY also contain an @type,
an @language, an @index, or an @context key but MUST NOT contain
both an @type and an @language key at the same time.
A value objectMUST NOT contain any other keys that expand to an
absolute IRI or keyword.

8.4 Lists and Sets

A list represents an ordered set of values. A set
represents an unordered set of values. Unless otherwise specified,
arrays are unordered in JSON-LD. As such, the
@set keyword, when used in the body of a JSON-LD document,
represents just syntactic sugar which is optimized away when processing the document.
However, it is very helpful when used within the context of a document. Values
of terms associated with an @set or @list container
will always be represented in the form of an array when a document
is processed—even if there is just a single value that would otherwise be optimized to
a non-array form in compact document form.
This simplifies post-processing of the data as the data is always in a
deterministic form.

A set objectMUST be a JSON object that contains no
keys that expand to an absolute IRI or keyword other
than @list, @context, and @index.
Please note that the @index key will be ignored when being processed.

In both cases, the value associated with the keys @list and @setMUST be one of the following types:

8.5 Language Maps

A language map is used to associate a language with a value in a
way that allows easy programmatic access. A language map may be
used as a term value within a node object if the term is defined
with @container set to @language. The keys of a
language mapMUST be strings representing
[BCP47] language codes and the values MUST be any of the following types:

8.6 Index Maps

An index map allows keys that have no semantic meaning,
but should be preserved regardless, to be used in JSON-LD documents.
An index map may
be used as a term value within a node object if the
term is defined with @container set to @index.
The values of the members of an index mapMUST be one
of the following types:

If the expanded term definition contains the @containerkeyword, its value MUST be either @list, @set,
@language, @index, or be null. If the value
is @language, when the term is used outside of the
@context, the associated value MUST be a language map.
If the value is @index, when the term is used outside of
the @context, the associated value MUST be an
index map.

TermsMUST NOT be used in a circular manner. That is,
the definition of a term cannot depend on the definition of another term if that other
term also depends on the first term.

9. Relationship to RDF

JSON-LD is a
concrete RDF syntax
as described in [RDF11-CONCEPTS]. Hence, a JSON-LD document is both an
RDF document and a JSON document and correspondingly represents an
instance of an RDF data model. However, JSON-LD also extends the RDF data
model to optionally allow JSON-LD to serialize
Generalized RDF Datasets.
The JSON-LD extensions to the RDF data model are:

Summarized, these differences mean that JSON-LD is capable of serializing any RDF
graph or dataset and most, but not all, JSON-LD documents can be directly
interpreted as RDF as described in RDF 1.1 Concepts [RDF11-CONCEPTS].

For authors and developers working with blank nodes
as properties when deserializing to RDF,
three potential approaches are suggested:

If the author is not yet ready to commit to a stable IRI, the
property should be mapped to an IRI that is documented as unstable.

The normative algorithms for interpreting JSON-LD as RDF and serializing
RDF as JSON-LD are specified in the JSON-LD Processing Algorithms and API
specification [JSON-LD-API].

Even though JSON-LD serializes
generalized RDF Datasets, it can
also be used as a RDF graph source.
In that case, a consumer MUST only use the default graph and ignore all named graphs.
This allows servers to expose data in languages such as Turtle and JSON-LD
using content negotiation.

Note

Publishers supporting both dataset and graph syntaxes have to ensure that
the primary data is stored in the default graph to enable consumers that do not support
datasets to process the information.

9.1 Serializing/Deserializing RDF

This section is non-normative.

The process of serializing RDF as JSON-LD and deserializing JSON-LD to RDF
depends on executing the algorithms defined in
RDF Serialization-Deserialization Algorithms
in the JSON-LD Processing Algorithms and API specification [JSON-LD-API].
It is beyond the scope of this document to detail these algorithms any further,
but a summary of the necessary operations is provided to illustrate the process.

The procedure to deserialize a JSON-LD document to RDF involves the
following steps:

Expand the JSON-LD document, removing any context; this ensures
that properties, types, and values are given their full representation
as IRIs and expanded values. Expansion
is discussed further in section 6.17 Expanded Document Form.

The process of serializing RDF as JSON-LD can be thought of as the
inverse of this last step, creating an expanded JSON-LD document closely
matching the triples from RDF, using a single node object
for all triples having a common subject, and a single property
for those triples also having a common predicate.

A. Relationship to Other Linked Data Formats

This section is non-normative.

The JSON-LD examples below demonstrate how JSON-LD can be used to
express semantic data marked up in other linked data formats such as Turtle,
RDFa, Microformats, and Microdata. These sections are merely provided as
evidence that JSON-LD is very flexible in what it can express across different
Linked Data approaches.

A.1 Turtle

This section is non-normative.

The following are examples of transforming RDF expressed in Turtle [TURTLE]
into JSON-LD.

Prefix definitions

This section is non-normative.

The JSON-LD context has direct equivalents for the Turtle
@prefix declaration:

Conversion of native data types

In JSON-LD numbers and boolean values are native data types. While Turtle
has a shorthand syntax to express such values, RDF's abstract syntax requires
that numbers and boolean values are represented as typed literals. Thus,
to allow full round-tripping, the JSON-LD Processing Algorithms and API specification [JSON-LD-API]
defines conversion rules between JSON-LD's native data types and RDF's
counterparts. Numbers without fractions are
converted to xsd:integer-typed literals, numbers with fractions
to xsd:double-typed literals and the two boolean values
true and false to a xsd:boolean-typed
literal. All typed literals are in canonical lexical form.

The representation of the hCard expresses the Microformat terms in the
context and uses them directly for the url and fn
properties. Also note that the Microformat to JSON-LD processor has
generated the proper URL type for http://tantek.com/.

B. IANA Considerations

This section has been submitted to the Internet Engineering Steering
Group (IESG) for review, approval, and registration with IANA.

application/ld+json

Type name:

application

Subtype name:

ld+json

Required parameters:

None

Optional parameters:

profile

A a non-empty list of space-separated URIs identifying specific
constraints or conventions that apply to a JSON-LD document according [RFC6906].
A profile does not change the semantics of the resource representation
when processed without profile knowledge, so that clients both with
and without knowledge of a profiled resource can safely use the same
representation. The profile parameter MAY be used by
clients to express their preferences in the content negotiation process.
If the profile parameter is given, a server SHOULD return a document that
honors the profiles in the list which are recognized by the server.
It is RECOMMENDED that profile URIs are dereferenceable and provide
useful documentation at that URI. For more information and background
please refer to [RFC6906].

This specification defines three values for the profile parameter.
To request or specify expanded JSON-LD document form,
the URI http://www.w3.org/ns/json-ld#expandedSHOULD be used.
To request or specify compacted JSON-LD document form,
the URI http://www.w3.org/ns/json-ld#compactedSHOULD be used.
To request or specify flattened JSON-LD document form,
the URI http://www.w3.org/ns/json-ld#flattenedSHOULD be used.
Please note that, according [HTTP11], the value of the profile
parameter has to be enclosed in quotes (") because it contains
special characters and, if multiple profiles are combined, whitespace.

When processing the "profile" media type parameter, it is important to
note that its value contains one or more URIs and not IRIs. In some cases
it might therefore be necessary to convert between IRIs and URIs as specified in
section 3 Relationship between IRIs and URIs
of [RFC3987].

Since JSON-LD is intended to be a pure data exchange format for
directed graphs, the serialization SHOULD NOT be passed through a
code execution mechanism such as JavaScript's eval()
function to be parsed. An (invalid) document may contain code that,
when executed, could lead to unexpected side effects compromising
the security of a system.

When processing JSON-LD documents, links to remote contexts are
typically followed automatically, resulting in the transfer of files
without the explicit request of the user for each one. If remote
contexts are served by third parties, it may allow them to gather
usage patterns or similar information leading to privacy concerns.
Specific implementations, such as the API defined in the
JSON-LD Processing Algorithms and API specification [JSON-LD-API],
may provide fine-grained mechanisms to control this behavior.

JSON-LD contexts that are loaded from the Web over non-secure connections,
such as HTTP, run the risk of being altered by an attacker such that
they may modify the JSON-LD active context in a way that
could compromise security. It is advised that any application that
depends on a remote context for mission critical purposes vet and
cache the remote context before allowing the system to use it.

Given that JSON-LD allows the substitution of long IRIs with short terms,
JSON-LD documents may expand considerably when processed and, in the worst case,
the resulting data might consume all of the recipient's resources. Applications
should treat any data with due skepticism.

Interoperability considerations:

Not Applicable

Published specification:

http://www.w3.org/TR/json-ld

Applications that use this media type:

Any programming environment that requires the exchange of
directed graphs. Implementations of JSON-LD have been created for
JavaScript, Python, Ruby, PHP, and C++.

C. Acknowledgements

This section is non-normative.

The authors would like to extend a deep appreciation and the most sincere
thanks to Mark Birbeck, who contributed foundational concepts
to JSON-LD via his work on RDFj. JSON-LD uses a number of core concepts
introduced in RDFj, such as the context as a mechanism to provide an
environment for interpreting JSON data. Mark had also been very involved in
the work on RDFa as well. RDFj built upon that work. JSON-LD exists
because of the work and ideas he started nearly a decade ago in 2004.

A large amount of thanks goes out to the JSON-LD Community Group
participants who worked through many of the technical issues on the mailing
list and the weekly telecons - of special mention are François Daoust,
Stéphane Corlosquet, Lin Clark, and Zdenko 'Denny' Vrandečić.

The work of David I. Lehn and Mike Johnson are appreciated for
reviewing, and performing several early implementations
of the specification. Thanks also to Ian Davis for this work on RDF/JSON.